Improve command-line argument handling of buildhistory-diff to make it
easier to use.

* Default buildhistory directory to buildhistory/ under the current
  directory and require an option to set it (since most users will
  likely run buildhistory-diff from the build directory and keep
  BUILDHISTORY_DIR at its default location)
* Default from-revision to "build-minus-1" to get the difference from
  the previous build with no arguments
* Allow from/to revisions to be specified by from..to (since git accepts
  this form).

(From OE-Core rev: 5e2be70e89820ffc74208d225fe4414fe5182050)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>